Getting a Better View: Quality of Today's Newtonian Optics
Whether you're in the market for a complete Newtonian telescope or just a set of mirrors, optical quality is a consideration. Some mirror makers, such as Carl Zambuto and Robert Royce, have attained almost legendary status for the quality of their mirrors. And as a general rule, the higher the accuracy of the mirrors, the more they will cost. But this begs two questions:
- What do the accuracy specs really mean?
- And can you detect a difference between mirrors of various quality levels? To address these issues, I've culled through some articles by pundits on telescope optics to see what they have to say.
In his book Star Ware 3, Philip Harrington explains the fine points of optical quality: "For a lens or a mirror to be accurate to, say, ? wave (a value frequently quoted by telescope manufacturers), its surface shape cannot deviate from perfection by more than 0.000069 mm, or 0.000003 inch! This means that none of the little irregularities (commonly called hills and valleys) on the optical surface exceed a height or depth of ? of the wavelength of yellow-green light. As you can see, the smaller the fraction, the better the optics. Given the same aperture and conditions, telescope 'A' with a ? wave prime optic (lens or mirror) should outperform telescope 'B' with a 1/4 wave lens or mirror, while both should be exceeded by telescope 'C' with a 1/20-wave prime optic."
The most important consideration is the final wavefront - the quality of the optics as seen by the observer's eye. For a Newtonian system, you have to take into consideration not only the primary mirror, but also the secondary mirror. Thus, if each mirror is corrected to ? wave, the optical system is considered to have a final wavefront of 1/4 wave (double the wave error of each mirror alone). A system error of 1/4 wave is considered the lowest level that will produce acceptable images; it's referred to as "Rayleigh's Criterion", and is what mirror suppliers usually refer to as "diffraction limited". But as a practical matter, those terms may tell you relatively little about the quality of the optical system. Read on.
If a diagonal mirror has errors of its own, it will distort the incoming wavefront, so even a good primary mirror couldn't produce acceptable images. On the other hand, highly corrected secondary mirrors are sometimes used to improve the final wavefront. Newtonian secondaries are flat mirrors, so it's easier to grind them to a high level of accuracy. But some have questioned the advantage of very highly corrected (and expensive) secondaries. According to telescope making author Mel Bartels, "When you get very close to the focal plane, errors can be tolerated, because now you are looking more from the perspective of the eyepiece; namely, what errors in the focal plane can the eyepiece tolerate and still project an acceptable image into the eye."
One source of confusion is that manufacturers don't all use a uniform criterion. When the astronomy magazines have tested optics, they've discovered that some quality claims were highly inflated. Harrington points out that "...many companies have dropped claims of their optics' wavefront, referring to them instead as 'diffraction limited', meaning that the optics are so good that performance is limited only by the wave properties of light itself and not by any flaws in optical accuracy. In general, to be diffraction limited, an instrument's final wavefront must be at least 1/4 wave, the Rayleigh Criterion. Once again, however, this can prove to be a subjective claim."
Be suspicious of claims that include "RMS" error or "wavelength of He/Ne (helium-neon) laser". Either one of those measures can make the optic seem better than it really is. A mirror maker should be able to tell you the peak-to-valley error in yellow-green light of the primary mirror itself. Less than ? wave is a no-no. The best opticians now use the Strehl ratio rather than wavefront error (a Strehl ratio of 0.8 is good, 0.9 is excellent, 0.95 or better is outstanding).
But can your eye easily detect a difference between optical systems of different wavefront accuracy? Implied in that is the issue of whether it's worth spending more money for premium optics. In tests that have been done by knowledgeable people, the difference from one set of optics to the next step up is so slight that most observers don't notice it. But when going from a scope that merely meets the Rayleigh Criterion to a set of exquisite optics, there is a noticeable difference. Where this shows up most is when viewing objects that have many low contrast details. But there is a limit. Carl Zambuto has said that the human eye can notice a difference up to 1/15 wave while observing, but anything above that smoothness will be lost due to atmospheric conditions.
A few years ago optical gurus Terry Dickinson, Peter Ceravolo, and Doug George conducted an experiment in which Ceravolo made a set of 6" f/8 Newtonians, identical except for the wavefront error of their mirrors. Many observers were invited to try these, and were consistently able to spot the ones made to 1/2 wave or poorer, but only very experienced observers could detect the subtle differences between 1/4 wave and 1/10 wave. Their conclusion was that most amateurs would be perfectly happy with the views from any scope that was an honest 1/4 wavefront or better.
Alan Rahill, a Canadian meteorologist and mirror maker, put a practical spin on it: "... I try not to wave rate mirrors but it seems to always pop up ... I'm guilty of guessing occasionally but usually try to limit it to worse, about, or maybe better than 1/4 wave ... I must say that my all time favorite 'wave-rating' is when a friend at a star party 'waves' and says 'Hey! You have got to come see through this scope over here!'"
Nonetheless, Rahill does apply the traditional wave rating when directly comparing mirrors by different manufacturers. Here's what he says: "I have tested about 50 mirrors in the last 5 years. Many Meade 8-10" and all were 1/4 - ? wave range. The 10" were particularly good... 1/6 - 1/8 wave! I tested Zambuto and they are really exceptional ... better than 1/10 wave. I tested two Swayze and they were about 1/5 wave with micro ripples. I tested two 20" galaxy mirrors at 1/3 wave. Surprise! The best mirrors I tested recently were Nova and Zambuto. Very smooth and no turned down errors, no zones ... really nice."
With budget constraints and a choice between a larger telescope with average optics or a smaller one with superb optics, which way should you lean? The larger the mirror the greater the resolution ... and light grasp is, after all, a major consideration. But to see detail in many objects requires contrast, and the better the mirror quality, the higher the contrast. This is especially true for objects such as the moon and planets.
Many of the relatively inexpensive mass-produced scopes imported recently from Guan Sheng of Taiwan and Synta of China have tested out to have surprisingly good mirrors. In some cases, it would take an astute observer to notice the difference between, let's say, an Orion XT series and a scope with a premium mirror. An advantage of a mirror by someone like Zambuto is consistency - you know every one will be of the absolute highest quality. This is something that the mirror makers in the Far East have yet to accomplish, although they've been steadily improving. All commercially made mirrors are ground by machine; it's the polishing and final figuring operation that makes the biggest difference. Premium mirror makers devote a great deal of time to this final step. That would prove far too costly for mass producers. Yet the fact that some of the mirrors from the Far East have tested out so well suggests that they've fine-tuned their mass production techniques.
So to get a telescope with as good a set of optics as you can afford, it's best to find out what actual users have to say. The Yahoo equipment-oriented chat groups are a source. Also check some of the Internet sites that offer product reviews. Two good ones are "Cloudy Nights": http://www.cloudynights.com/ and Ed Ting's "Telescope Review Web Site": http://www.scopereviews.com/
You may be very happy with one of the bargain-priced mass-produced telescopes, provided you check out the optical quality when you first receive it and demand a replacement if it presents unsatisfactory images. Most telescope reviewers agree that the general optical quality of Newtonian telescopes now on the market is quite good - actually somewhat better than it was just a few years ago. Good optics don't waste light by scattering it ... they let you see every photon that makes its way to your telescope.Published in the June 2006 issue of the NightTimes